The President's Address. By A. D. Michael. 
33 
Finally, let us glance cursorily at the great question of pliylogeny ; 
it can only be very cursorily, for time is short. Whence have the 
Acarina sprung? Many eminent biologists have lately been of 
opinion that they are degraded Arachnida ; this appears a very probable 
theory, but there seem to me to be certain difficulties in the way of its 
immediate acceptance. If the Acari were in course of degradation 
then we should expect, in accordance with Fritz Muller’s well-known 
law that the ontogeny includes the phylogeny, that the larval forms 
would be more highly organized than the adults ; but the exact con- 
trary is the fact ; in every case where there is a difference the adult 
is the more highly organized. The Tyroglyphidse are about the 
lowest forms of free-living Acarina ; they have no tracheae or special 
breathing-organs of any sort ; the adult Oribatidae have unbranched 
tracheae in much the same condition as those of Peripatus, only that 
their number and the position of their stigmata is fixed ; but The im- 
mature Oribatidae are entirely devoid of tracheae, so that Claparede 
expressed it that they passed through a Tyroglyphus-stage. In 
Sphderogyra the larval forms are entirely without tracheae and those 
organs exist only in the adult female ; while in other Trombidiidae 
both sexes have richly branched tracheae. Adult Halicaridae,^ which 
inhabit shallow seas, i.e. the littoral zone, are said by Lohmann to 
show traces of tracheal folds near the mouth ; and examples might be 
multiplied. Again, the analogy to the Insecta has been pressed, and it 
has been suggested that the four pairs of legs represent the legs of 
an insect plus the absent labial palpi ; but almost all Acari in their 
first larval condition have three pairs of legs only, and how does this 
agree with the “ insects’ limbs ” theory ? The pair that developes after 
the first change of skin is the fourth pair. It is true that Winkler 
announced the surprising discovery that in the early embryology of 
Gamasus four pairs of legs were developed, one of which atrophied 
before birth, to be again developed at the first larval ecdysis — a fact 
which, extraordinary as it is, is not entirely without a parallel in nature ; 
but as far as it has any bearing it would speak equally in favour of a 
relationship to other Arachnida. I have indicated before the difficulty 
of finding a sufficiently clear distinction between Phalangiidae and 
Acarina ; if a puzzling intermediate form occurs in the Acarine border- 
land it is sure to be on the Phcdangium border. It seems to me 
that, as far as our present knowledge goes, Phalangium is the nearest 
form morphologically, but how that resemblance has been brought 
about is a different matter. Shall we ever be able really to decide 
these points? that is a question which it is beyond my power to 
answer ; if it ever be done it will probably be when both this 
somewhat rambling address and its author have been long forgotten. 
1894 
D 
